As is well known in the art, the degree of integration in semiconductor integrated circuits has marked remarkable progresses in recent years. In unison with this tendency, the light source used in the lithography process for the fabrication of semiconductor devices has achieved a substantial reduction of exposure wavelength. The current mainstream lithography uses an ArF excimer laser of 193 nm wavelength. In the drive to achieve a higher degree of integration, the lithography using ArF excimer laser may survive in further developed forms like immersion lithography and double patterning lithography, and a subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising.
The EUV lithography is expected to use a light source of soft x-ray having a wavelength of up to 70 nm, specifically around 13 nm. Since there are no materials which are highly transmissive in this wavelength range, the EUV lithography has to employ a reflecting optical system. While reflection occurs in this system by a reflective multi-layer film of silicon, molybdenum or the like deposited on a substrate, several tens percents of incident EUV light will not be reflected and reach the underlying substrate where it transforms into heat. Since the EUV lithography uses an extremely short wavelength light source as compared with the conventional lithography technology, the lithography precision can be adversely affected even by a slight thermal expansion of each member (e.g., substrate) in the lithographic optical system induced by the heat that has reached there. Accordingly, members like reflecting mirrors, masks, and stages must be made of low expansion materials. Titania-doped quartz glass is a typical low expansion material. The addition of a certain amount of titania makes it possible to minimize the thermal expansion of quartz glass.
In the EUV lithography with a very short wavelength light source, since the lithography precision can be adversely affected even by slight irregularities on the member surface, a high precision is required of the surface morphology. However, conventional polishing methods often fail to provide a surface morphology of high precision needed as the EUV lithography members.
Besides the conventional polishing methods, a method of providing a surface morphology of high precision, for example, high flatness is disclosed in JP-A 2006-8426 which relates to local ion beam etching on a member surface. U.S. Pat. No. 6,855,908 (EP 1251108 A1 or JP-A 2002-316835) discloses plasma etching on a member surface. However, these etching methods cause remarkable increases in the manufacture cost and retards in the manufacture time of members. There is thus a desire to manufacture members having a high surface precision necessary for EUV lithography members, using only conventional polishing methods, without resorting to the etching methods.